As rolls and the like to be used in a plating bath of a molten zinc plating line or a molten zinc-aluminum plating line, there have been used members obtained by spraying various cermet materials or oxide ceramic materials on a surface of a thermal resistant steel roll.
Cermet sprayed coatings applied on the surfaces of the steel member have, however, such disadvantages that corrosion resistance against a molten metal is poor and that the ceramic sprayed coatings may be easily peeled off.
As means to solve the above-mentioned problems, there has been proposed in JP-A-5-209259 a method for spraying a cermet material containing 5-60% of metal boride, 5-30% of one or more member(s) selected from the group consisting of Co, Cr, Mo and W, as well as the balance comprising carbide and unavoidable impurities on a surface of a steel member and spraying thereon an oxide ceramic. Cr.sub.2 O.sub.3 is mentioned therein as an example of the oxide ceramics. Although properties thereof have been improved in use and good results have been attained, more absolute means have been required for further improvement.
On the other hand, there has been proposed in JP-A-4-350154 a sprayed coating having two-layer constitution in which an oxide ceramic sprayed layer containing SiO.sub.2 and the balance consisting of at least one member selected from the group consisting of MgO, CaO, ZrO.sub.2, Al.sub.2 O.sub.3, Y.sub.2 O.sub.3 and TiO.sub.2 is arranged on a lower layer of a carbide cermet sprayed layer containing one or more carbide(s) and one or more metal(s) selected from the group consisting of Co, Ni, Cr and Mo.
That is, in the case that the lower layer is a carbide cermet, fine cracks for absorbing thermal stress can be produced in the upper ceramic layer by containing 10-40% by weight of SiO.sub.2 in the upper ceramic layer. It is explained therein that the sprayed coating is effective as a member for a molten metal bath.
It is, however, required to produce fine cracks in the ceramic layer (the upper layer), since the method has a prerequisite of using the carbide cermet as the lower layer. Furthermore, thermal impact resistance is improved but stability in quality against a molten metal becomes poor, since corrosion resistance and wetting resistance against a molten metal are influenced by an extent of fine longitudinal cracks.